1. Field of the Invention
This invention relates to alloy steel powders for manufacturing iron sintered bodies requiring high strength and high compressibility. It further relates to high strength, high compressibility sintered bodies produced, and to a method of manufacturing the sintered bodies.
2. Description of the Related Art
When iron parts requiring high strength are manufactured by conventional powder metallurgy, alloy steel powders are compacted with added strength-enhancing alloy element powders such as Ni, Cu, Mo, Cr and the like. Alternatively, this is done using alloy steel powders made by adding such strength-enhancing alloy elements to molten steel, sintering these alloy steel powders, then carburizing and nitriding and thereafter quenching and tempering the resulting alloy steel powders. Further repeating compacting and sintering of the alloy steel powders, after the first sintering, may be practiced to obtain high strength. It is inevitable, however, that the repetition of the heat treatment and compacting steps increases manufacturing cost. Further, repetition of heat treatment reduces dimensional accuracy of the resulting sintered body.
For example, Cr--Mn alloy steel powders capable of obtaining high strength and exhibiting excellent hardenability are examples of sintered and heat-treated materials whose strength is improved by the addition of strengthening elements (such as Cr) with molten steel (Japanese Patent Publication No. 58(1983)-10962). However, Cr and Mn lower compressibility when powder particles are hardened and compacted, thus shortening the life of a mold. Additional drawbacks include cost increases caused by heat treatments such as quenching, tempering and the like in the manufacturing of steel powders and low dimensional accuracy from the repetition of heat treatments.
Through extensive study, we have discovered remarkable steel powders which can achieve high strength and excellent compressibility after a single sintering operation (omitting the above-described heat treatment). The inventors have proposed Japanese Patent Application Laid-Open No. Hei 4(1992)-165002 and Japanese Patent Application Laid-Open No. Hei 5(1993)-287452 based on these discoveries.
Japanese Patent Application Laid-Open No. Hei 4(1992)-165002 increases the strength of a sintered body by adding Nb and V to Cr alloy powders and utilizing a carbide and nitride precipitation mechanism such that the content of Mn is reduced. Since the powders contain only 0.005-0.08 wt % of V, however, the strengthening effect of the carbides and nitrides of V is lessened. Further, since a large amount of Mo (0.5-4.5 wt %) is used to improve the strength of the sintered body, coarse upper bainite is produced causing the strength of the resulting sintered body to be lower than that of a heat-treated body.
Japanese Patent Application Laid-Open No. 5(1993)-287452 improves strength and fatigue strength by reducing the number of sites of fracture caused by oxide and the like. This is accomplished by further reducing the contents of Mn, P, S in conventional Cr alloy steel powders and limiting the cooling rate after sintering, thereby creating a fine pearlite structure in the sintered body. However, such alloy steel powders are sensitive to the cooling rate after sintering such that the strength of the sintered body is greatly dispersed depending upon the cooling rate. Thus, it is difficult for users to handle these alloy steel powders.